In the development of an electroluminescent device utilizing an organic material (hereinafter referred to as an organic EL device), a device developed by optimizing the kind of electrodes and disposing a hole-transporting layer comprising an aromatic amine and a light-emitting layer comprising 8-hydroxyquinoline aluminum complex as thin films between the electrodes for the purpose of improving the efficiency of injecting electrical charges from the electrodes has brought about a noticeable improvement in luminous efficiency over the conventional devices utilizing single crystals of anthracene and the like. Since then, the developmental works of organic electroluminescent devices have been focused on their commercial applications to high-performance flat panels characterized by self luminescence and high-speed response.
The aforementioned device comprising an aromatic diamine in the hole-transporting layer and 8-hydroxyquinoline aluminum complex in the light-emitting layer and many others utilize fluorescence. However, utilization of phosphorescence, that is, emission of light from the triplet excited state, is expected to raise the luminous efficiency three to four times that of the conventional devices utilizing fluorescence (singlet). For this reason, a large number of developmental works are directed to phosphorescent dopants.                Patent document 1: JP2003-515897 A        Patent document 2: JP2001-313178 A        Patent document 3: JP2002-352957 A        Non-patent document 1: Nature, Vol. 395, p. 151, 1998        Non-patent document 2: Appl. Phys. Lett., Vol. 75, p. 4, 1999        
The non-patent document 1 reports that the use of a platinum complex (PtOEP) enables the device to emit red light at high efficiency. Thereafter, the non-patent document 2 reports that the efficiency of emission of green light improves appreciably by doping the light-emitting layer with an iridium complex or tris(2-phenylpyridine)iridium [Ir(ppy)3]. This document further reports that optimization of the light-emitting layer enables this iridium complex to show extremely high luminous efficiency even when the structure of the device is simplified still further.
In applications of organic EL devices to display devices such as flat panel displays, it is necessary to improve the luminous efficiency of the device and, at the same time, to fully secure the driving stability of the device. However, the organic electroluminescent device described in the non-patent document 2, that is, a device which utilizes a phosphorescent molecule and shows a high luminous efficiency, lacks sufficient driving stability for practical use at the present time.
In the non-patent document 2, 4,4′-bis(9-carbazolyl)biphenyl (CBP) or 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (TAZ) is used in the light-emitting layer and a phenanthroline derivative is used in the hole-blocking layer. Moreover, CBP is described as a preferable host material in the non-patent documents 1 and 2.
However, as the aforementioned CBP has a property of facilitating the flow of holes and obstructing the flow of electrons, the use of CBP destroys the balance of electrical charges in the light-emitting layer and allows excess holes to flow out to the side of electron-transporting layer and, as a result, the luminous efficiency of Ir(ppy)3 drops. On the other hand, TAZ has a property of facilitating the flow of electrons and obstructing the flow of holes and the use of TAZ shifts the light-emitting range to the side of the hole-transporting layer. In this case, the hole-transporting material in use affects the luminous efficiency of Ir(ppy)3. For example, the use of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) which is most often used in the hole-transporting layer for its high performance, high reliability, and long service life causes transition of energy to occur from Ir(ppy)3 to NPB and the luminous efficiency drops.
As a means to solve the aforementioned problem, the use of 4,4′-bis[N,N′-(3-toluoyl)amino]-3,3′-dimethylbiphenyl (HMTPD) to which transition of energy from Ir(ppy)3 does not occur is conceivable, but this compound is not considered sufficiently durable.
Furthermore, CBP and TAZ readily undergo crystallization and cohesion thereby deteriorating the shape of their thin films and, besides, it is difficult to merely observe the Tg because of their high crystallinity. The instability of the shape of thin film inside the light-emitting layer adversely affects the device by shortening the driving life and lowering the heat resistance. For the aforementioned reasons, the largest problem facing the organic electroluminescent devices utilizing phosphorescence is how to secure the driving stability of the device.